1. Field of the Invention
This invention relates generally to a thermal switch including a bimetal or trimetal thermally responsive element, and more particularly to such a thermal switch provided in, for example, an automobile to detect a temperature of equipment such as a compressor circulating refrigerant to a heat exchange system or an engine transmission for the purpose of protecting the equipment against an overheat or an overcurrent in an abnormal condition.
2. Description of the prior art
There have conventionally been provided thermal switches of the above-described type which open and close an electric circuit using deformation of a bimetal or trimetal. FIG. 8 illustrates one of the conventional thermal switches. The thermal switch 101 comprises a generally circular metal header plate 102 and a cylindrical bottomed housing 103. An open end of the housing 103 is airtightly welded or otherwise secured to the header plate 102, so that a hermetic housing is constituted by the header plate 102 and the cylindrical housing 103. The hermetic housing is used to prevent penetration of water etc. thereinto and to maintain a stable state of composition of a gas filling an interior thereof for a long period. The cylindrical housing 103 is made of a steel plate which provides a good veldability.
The header plate 102 has two through holes 102A and 102B. Two electrically conductive metal terminal pins 104A and 104B are inserted through the respective holes 102A and 102B and hermetically secured in the respective holes by an electrically insulating filler 105 such as glass. A generally C-shaped thick electrically conductive fixed contact member 106 is welded or otherwise secured at its upper end to a lower end of one terminal pin 104A. An elastic movable contact member 107 has a fixed end 107A welded or otherwise secured to a lower end of the other terminal pin 104B. The movable contact member 107 further has a distal end 107B to which a movable contact 108 is secured so as to come into contact with a contact portion 106A of the fixed contact member 106.
A thermally responsive element 109 is made by punching a disc out of a material such a bimetal and forming the disc into the shape of a shallow dish. The thermally responsive element 109 is placed on the bottom of the housing 103, and a retainer 110 made of an elastic material is placed on the thermally responsive element 109. A pressure piece 111 is further provided on the retainer 110. The pressure piece 111 is made of an electrically and thermally resisting material and has a distal end force-fitted into a hole 107C formed in the movable contact member 107 to be fixed.
The thermally responsive element 109 of the thermal switch 101 is downwardly convex at a normal temperature. When the ambient temperature is increased to reach a predetermined value, the thermally responsive element 109 reverses its curvature with snap action so as to be upwardly convex. An upwardly convex central portion pushes the pressure piece 111 upward. The pressure piece 111 then pushes the movable contact member 107 upward so that the distal movable contact 108 is disengaged from the contact portion 106A of the fixed contact member 106, whereupon an electric circuit between the terminal pins 104 and 104B is broken.
FIGS. 9 and 10 show the above-described thermal switch 101 mounted on a compressor for a car air conditioner for the purpose of protecting the compressor against the overheat or the overcurrent. The compressor comprises a casing A previously provided with a mounting section A1 which is a through hole open to a passage of discharged refrigerant. The mounting section A1 is positioned so that the thermal switch 101, when mounted thereon, can quickly detect a temperature of the refrigerant.
Lead wires 112A and 112B are connected to the terminal pins 104A and 104B of the thermal switch 101 respectively. A protective cap 113 is put onto the thermal switch 101 in order that water etc. may be prevented from penetrating connections between the terminal pins and the lead wires during the service and so that the thermal switch 101 may be protected against an external force or vibration applied thereto during the mounting of the thermal switch. For these purposes, too, the mounting section is filled with an insulating filler 114. The thermal switch 101 is inserted into the mounting section A1 together with an O-ring 115 made of silicon rubber or the like. A known arcuate elastic member 116 such as a snap ring is attached to an upper peripheral end of the protective cap 113 to hold the latter, whereby the mounting section A1 is closed by the thermal switch.
In the above-described thermal switch, the thermally responsive element 109 is positioned on the bottom of the housing 103 so that a high thermal responsiveness can be achieved. On the other hand, smaller thermal switches have recently been desired in view of a problem of the position where the thermal switch is mounted. However, when the thermal switch is mounted on the casing of the car air conditioner compressor which is usually exposed to outside air, beat of the surface of the compressor casing is absorbed into the outside air. Accordingly, heat of the thermal switch mounted directly on the compressor casing is absorbed into the outside air and by heat conduction through the compressor casing. Particularly when the outside air temperature is low, the above-described thermal switch cannot provide a sufficient thermal responsiveness for a rapid increase in the refrigerant temperature. Further, the header plate 102 necessitates a sufficient thickness for holding the terminal pins 104A and 104B. This results in are increase in the heat capacity of the header plate 102. As a result, heat at the distal end of the housing is absorbed via its cylindrical portion into the header plate 102. This reduces a temperature increasing speed of the thermally responsive element 109 and accordingly, the responsiveness of the thermal switch. Particularly when the housing is rendered smaller, the length of the cylindrical portion of the housing is also reduced, so that heat tends to be absorbed into the header plate side.
A mounting manner as shown in FIG. 11 is effective for increase in the response speed of the thermal switch. In this manner, the thermal switch 101 is fixed to a distal end of a terminal pin 122A of a closed terminal 122 electrically connecting between the exterior and the interior of the compressor 121. Thus, heat conduction from the thermal switch to the outside of the compressor can be minimized by locating the thermal switch inside an inner wall of the compressor casing only by means of the terminal pin, and the thermal responsiveness can be improved by exposing the overall thermal switch to the refrigerant as a heating medium or a heat carrier. An alternative characteristic experiment made by the inventors with use of oil instead of the heating medium shows that a response time is shortened to one half or less. The inventors have confirmed that the same result can be achieved in the actual use. In the above-described mounting manner, however, the refrigerant passage is required to have such a width that the thermal switch is accommodated therein. This requirement renders the compressor housing large-sized and increases the number of components of the compressor, resulting in an increase in the manufacturing cost of the system. Further, the above-described problem of reduction in the temperature increasing speed of the thermally responsive element due to the large heat capacity of the header plate still remains unsolved in this mounting manner.